In devices that clean the interior of tanks in chemical plants, food tanks and fermentation tanks, a nozzle is for example disposed inside the tank, and made to rotate so as to spray cleaning fluid inside the tank.
A device wherein a pair of nozzles are made to rotate about a horizontal axis, and the holder supporting the nozzles is made to rotate about a vertical axis so as to spray cleaning fluid uniformly in up, down, left and right directions, is disclosed in Tokko Sho 62-60146 published by the Japanese Patent Office.
However, if the tank is longer in the upward/downward direction as in the case of a cylindrical tank, for example, the tank cannot be cleaned uniformly if the nozzle is fixed even if the spray is made uniform in all directions. In such a tank, uniform cleaning can be performed if the nozzle holder is moved up and down while spraying takes place, but this requires the use of a raising and lowering mechanism comprising a motor and gears for example, that make the device complicated and costly.
In order to solve this problem, cleaning devices wherein the nozzle holder is moved up and down by the pressure of the cleaning fluid are disclosed in Tokkal Sho 59-203679, Tokko Sho 56-20907 and Tokko Hei 4-59034 published by the Japanese
These devices employ a cylinder housing a piston that elongates and contracts a piston rod fixed to the piston due to the pressure of the cleaning fluid, the nozzle holder being supported at the end of this rod. The cylinder comprises pressure chambers separated by the piston that drive the piston to elongate or contract the piston rod. When pressurized fluid is selectively supplied to one or the other of these chambers, therefore, the nozzle holder rises or falls. The cylinder is attached on the top of the tank, and as the nozzle holder rises or fails in the tank due to the elongation or contraction of the piston rod, cleaning fluid is sprayed from the nozzle. This cleaning fluid is supplied through the hollow part of the piston rod. When it is not in use the nozzle holder is raised to its storage position at the top of the tank due to the contraction of the piston rod.
According to these devices, cleaning of long tanks, such as for example cylindrical tanks, may be uniformly performed without using electrical energy.
In these devices, however, if the piston rod elongates and contracts at a constant speed, the cleaning fluid that is sprayed due to the rotation of the nozzles and nozzle holder leaves marks at regular intervals on the inner surface of the tank. If the speed of the piston rod is slowed down, the interval between these marks becomes smaller and the cleaning efficiency improves, but more time is then required until the whole cleaning process is accomplished. In order to obtain high cleaning efficiency in a short time, it is therefore desirable for example to make the ascending speed of the piston rod different from its descending speed so that cleaning marks do not overlap each other.
Further, in order for the cleaning device to clean the tank automatically according to a preset program, the change-over of the piston rod from elongation to contraction and vice versa, the ascending/descending speeds of the nozzle holder stud the number of ascents and descents of the nozzle holder must be centrally controlled. However, these devices were not provided with a control mechanism to optimize cleaning efficiency and automate the cleaning operation.